Curiosity's autonomous 'seven minutes of terror' (pictures)

Curiosity's unassisted descent

As the Curiosity rover enters the thin Martian atmosphere on Sunday night it will have traveled a total distance of about 352 million miles on the latest NASA mission. Jettisoned from its Atlas V541 launch vehicle, Curiosity must make a completely unassisted descent and landing on Mars, traveling from 13,000 mph to 0 mph, without direct help from personnel on Earth.

NASA Engineer Adam Steltzner explains that it takes 14 minutes for the communications signals to be transmitted the distance from Earth to Mars, meaning once NASA gets confirmation Curiosity has entered Mars' atmosphere, the mission's fate has already been decided -- the rover will already either be safely sitting on Mars, or it will have been destroyed upon entry.

The critical entry, descent, and landing (EDL) maneuvers include a combination of technologies inherited from past NASA Mars missions, as well as exciting new technologies, NASA says. Instead of the familiar airbag landing used during past, far smaller and lighter Mars missions, Mars Science Laboratory will employ a parachute, landing rockets, a hovering sky crane, and other complicated mechanisms to help lower the rover to the surface of the Red Planet.

Targeted landing area

The Mars Science Laboratory science team divided up the location where the mission's rover, Curiosity, will land into a series of "quadrangles." This includes the targeted landing ellipse in red, and adjacent areas within Gale Crater.

More than 30 team members mapped the quadrangles, which show great diversity in their geological attributes, including: portions of an alluvial fan (quads 31, 32, 33); layered deposits (quad 50 and many others); dunes composed of dark gray sand (quads 92, 54, 28); the basal-layered deposits of Mount Sharp (quads 118, 107, 83); and buried impact craters (quad 81). Many of these features represent important targets in the search for habitable environments.

Atlas V541 launch vehicle

With the Mars Science Laboratory payload perched on top, seen here in an artist's concept, the Atlas V541 is a launch vehicle capable of lifting the massive 8,463 pound payload -- the largest payload ever delivered to the surface of a planet.

In the scene depicted here, the payload fairing that encloses the spacecraft during ascent through the atmosphere is being released. It's from this point the entry interface begins to run, the craft is without human assistance and will have to run through the next mission-critical steps to landing completely autonomously.

Separation from the Atlas V541 launch vehicle

After separation from the Atlas V541 launch vehicle, the Mars Science Laboratory spacecraft, with the rover Curiosity and descent stage, are tucked inside the aeroshell. At this point, as the Rover enters the atmosphere, still traveling at about 13,000 miles per hour, NASA will lose contact with the vehicle, and we begin what is known as the "seven minutes of terror" during which time the landing systems are automated, and all the NASA engineers back on Earth can do is cross their fingers and wait for a successful touchdown.

The Mars approach

The mission's approach phase begins 45 minutes before the spacecraft enters the Martian atmosphere. It lasts until the spacecraft enters the atmosphere. For navigation purposes, the atmospheric entry point is 2,188 miles above the center of the planet.

This illustration depicts a scene after the spacecraft's cruise stage has been jettisoned, which will occur 10 minutes before atmospheric entry.

Entry, descent, and landing

Using the stars to navigate, the cruise stage will perform several trajectory correction maneuvers during this time to adjust the spacecraft's path toward its final, precise landing site on Mars at the Gale Crater. The on-board propulsion system, consisting of eight thrusters to be fired on command using hydrazine fuel in two titanium tanks, will be adjusting the spacecraft's position relative to stars in our Milky Way galaxy.

At about 81 miles, the entry, descent, and landing (EDL) phase begins when the spacecraft reaches the Martian atmosphere. EDL maneuvers include a combination of technologies used during past NASA Mars missions, as well as new technologies. Instead of the familiar airbag landing of past Mars missions, Mars Science Laboratory will use a guided entry and a sky crane touchdown system to land the hypercapable, massive rover.

Safely inside the aeroshell's heat shield

During this approach, as the craft speeds through the atmosphere, the Curiosity rover and the descent stage are safely tucked inside the aeroshell's heat shield and backshell, depicted in this artist's rendering. The diameter of the aeroshell is 14.8 feet, the largest ever used for a mission to Mars.

Careening through the Martian atmosphere, the shell will be heated to more than 1,600 degrees Celsius by friction, which will also slow the craft significantly, to 1,000 mph. This, however, is still faster than the speed of sound, and far too fast to allow for a safe landing. The Martian atmosphere poses engineering challenges for NASA -- being 100 times thinner than Earth's, it's thick enough to destroy an improperly shielded spacecraft during entry, but not thick enough to slow the craft to subsonic speeds.

Parachute to powered descent

To meet the atmospheric challenges of safe entry, NASA designed the largest and strongest supersonic parachute ever created, weighing only 100 pounds but capable of withstanding more than 65,000 pounds of force. The parachute is deployed with 9G's of force, and the heat shield is detached, allowing the instruments to get accurate navigational measurements to complete the landing.

The parachute will slow the craft significantly -- to about 200 miles per hour, but not enough to touch down safely, so NASA added a third stage of descent assistance: a powered descent.

Slowed by retro-rockets

Once the parachute is jettisoned, the craft is slowed by retro-rockets, capable of vertical and horizontal movements that stabilize the rover and move it out of the way of the parachute so that it doesn't become tangled.

At this point, the rover begins using radar, and its cameras see the surface and spot the landing area, ensuring it makes a safe landing.

Lowered by the sky crane

The use of the rocket powered descent, however, posed still yet another challenge. NASA did not want the rocket powered craft to go all the way to the surface because of the possibility the jets might kick up dust and debris, potentially damaging the sensitive instruments on board.

The solution was the sky crane, a 21-foot tether which will safely lower the rover the final distance to the ground.

Touchdown ends the 7 minutes of terror

The rover then touches down and the line is immediately cut, and the descent stage flies up and away to a safe distance from Curiosity. Safely on the surface of Mars, Curiosity powers up and makes contact with Earth, ending the 7 minutes of terror.

NASA says the span of time from atmospheric entry until touchdown is not predetermined. Exact timing and altitude for key events depends on unpredictable factors in atmospheric conditions on landing day, and the decisions will be made by the spacecraft during the descent.

The guided entry technique enables the spacecraft to respond and adapt to the atmospheric conditions it encounters more effectively than any previous Mars mission.

Curiosity's target landing area

This image shows changes in the target landing area for Curiosity. The larger ellipse was the target area prior to early June 2012, when the project revised it to the smaller ellipse centered nearer to the foot of Mount Sharp, inside Gale Crater.

The larger ellipse, 12.4 miles by 15.5 miles, was already smaller than the landing target area for any previous Mars mission, owing to this mission's techniques for improved landing precision. Continuing analysis after the Nov. 26, 2011, launch resulted in confidence in landing within an even smaller area, about 12 miles by 4 miles.

Gale Crater on Mars

Gale Crater on Mars, where NASA's Curiosity rover is set to land on August 5, 2012, belongs to a family of large, very old craters shown here on this elevation map. It has one of the lowest elevations among this family.

The NASA TV Public Channel and http://www.ustream.tv/nasajpl will carry a feed including commentary and interviews from mission control rooms at JPL between 8:30 p.m. and 11:00 p.m. PDT on August 5 (11:30 p.m. August 5 to 2:00 a.m. August 6 EDT), and between 12:30 a.m. and 1:30 a.m. PDT on August 6 (3:30 a.m. to 4:30 a.m. EDT).